The invention relates to electrodes employed for electrically sensing or stimulating biological tissues. In particular, the invention relates to two dimensional electrode arrays and to methods for making and using such electrode arrays. The electrode array is particularly useful for making multiple electrical contacts at the cellular level, for electronically discriminating amongst individual cells or small groups of cells within a tissue or organ, and for directing electrical signals to or from such individual cells or small groups of cells within such tissue or organ, especially neural tissues and organs.
A nerve is a cordlike structure which is composed of numerous nerve fibers conveying impulses between a part of the central nervous system and some other region of the body. A nerve is made up of individual nerve fibers with their sheaths and supporting cells, small blood vessels, and a surrounding connective tissue sheath. Each nerve fiber is surrounded by a cellular sheath (neurilemma) from which it may or may not be separated by a laminated lipo-protein layer (myelin sheath). A group of such nerve fibers surrounded by a sheet of connective tissue (perineurium) is called a fasciculus. The fasciculi are then bound together by a thick layer of connective tissue (epineurium) to form the nerve.
Neurologists have long sought an electrode device which could establish stable electrical contact with a large number of individual nerve fibers within a nerve. Such a device would find wide medical application for sensing neurological impulses, facilitating the analysis and interpretation of such impulses, and delivering electrical stimuli to target nerve fibers as a reaction to such analysis or as a result of external input. The ideal electrode device would be adapted to the anatomy of the nerve so that it could penetrate the nerve in a nondestructive fashion in order to form focused electrical contacts with a very large number of individual nerve fibers.
Nerve cuff electrodes are employed in the neurological sciences for sensing nervous impulses and for electrically stimulating nerves The nerve cuff electrode encircles the entire nerve and senses gross nervous impulses arising from the nerve fibers within the nerve. The nerve cuff electrode may also be employed to electrically stimulate the nerve. Individual nerve fibers within a nerve may be functionally distinct from the other nerve fibers. The utility of the nerve cuff electrode is limited by its inability to specifically direct signals to or from selected nerve fibers within the nerve.
In order to make electrical contact with individual nerve fibers within a nerve, narrow gauge needle electrodes may be employed. When a narrow gauge needle is inserted into the nerve, there is a chance that it may make electrical contact with an individual nerve fiber or a small number of such fibers. If electrical contact is desired with each of several nerve fibers, then several needle electrodes must be employed. However, the technique of using multiple needle electrodes becomes progressively more and more difficult as the number of electrodes increases. Hence, there is a limit to the number of needle electrodes which can be usefully employed on a single nerve. Also, the electrical contact between a needle electrode and its corresponding nerve fiber can be disrupted by muscle motion and other forms of motion, since the end of the needle opposite the electrode extends outside the nerve and can be levered by relative motion of neighboring tissues. Therefore, long term implantation of needle electrodes with stable electrical contact with nerve fibers is not possible with prior art needle electrodes.
An electrode array having several electrodes integrated into one device is disclosed by Robert L. White. (Proceedings of the first International Conference on Electrical Stimulation of the Acoustic Nerve as a Treatment for Profound Sensorineural Deafness in Man, published by Velo-Bind, Inc. (1974), edited by Michael M. Merzenich, et al., chapter entitled "Integrated Circuits and Multiple Electrode Arrays," pp. 199-207, by Robert L. White) White's electrode array employs a prong shaped base fabricated from a silicon wafer. The silicon base supports an array of electrodes which are deposited thereon toward the end of the prong. Each of the electrodes is small, flat, and circular, about 50 micrometers in diameter. Each electrode is connected to a corresponding conductor which carries signals to and from the electrode. The conductor is electrically insulated from the tissue by a layer of silicon dioxide. In use, the prong is inserted tip first into neural tissue. Neural tissue is displaced by the prong as it is inserted. Substantial damage to neural tissue can result from the insertion process due to the relatively large bulk of the prong. Since neural tissue slides tangentially past the electrodes during the insertion process, the flatness of the electrodes helps to minimize the resultant disruption and destruction of neural tissue. However, once the device is inserted, the flatness of the electrodes limits the contact between the electrode and the neural tissue. Flat electrodes can make electrical contact only with neural tissue which is directly adjacent to the surface of the prong.
Multiple electrode devices with micro electrode tips protruding beyond and in a plane parallel to a silicon carrier (i.e. planar electrodes) are disclosed by Wise et al. (IEEE Transactions on Biomedical Engineering, Vol. BME-17(3), pp 238-247, July 1970, "An Integrated Circuit Approach to Extracellular Microelectrodes," and Vol. BME-22(3), May 1975, "A Low-Capacitance Multielectrode Probe for Use in Extracellular Neurophysiology") and by Ko (IEEE Transactions on Biomedical Engineering, Vol. BME-33, pp 153-162 (Feb. 1986), "Solid State Physical Transducers for Biomedical Research"). Wise et al. teach that the lateral spacing and length of the protruding tips may be controlled to produce various planar electrode arrays. Like the White device, the silicon carrier of the Wise et al. and Ko devices have the shape of a prong and may cause significant tissue damage to the nerve during the insertion process. Also, if the Wise et al. and Ko prong-shaped devices are implanted, their large bulk compromises the stability of the electrical contact between the electrode tips and individual target cells. Additionally, the thinness of the prong can make it susceptible to shear damage with side loading. Further, since the silicon carrier and the electrode tips are essentially coplanar with the tips cantilevered freely beyond the end of the carrier, the carrier imparts little if any transverse stability to the fragile tips during insertion of the Wise et al. and Ko prong-shaped devices or after their implantation. Moreover, the number of useful electrodes which may be incorporated into the Wise et al. and Ko devices is inherently limited. Moreover, since the electrode tips are aligned in a row along the edge of the silicon carrier, it is not possible to array the electrodes into a configuration with more than one dimension.
Thus, what is missing from the prior art and what is needed by practicing neurologists is an implantable electrode device which can electrically contact a large number of individual cells within an organ or tissue for sensing and controlling various bodily functions. The individual contacts should each be focused within a small region so that they involve single cells only. However, the range of the contacts should extend over a relatively large two or three dimensional region within the organ or tissue. The electrodes of the device should make positive contact with target cells and should be electrically stable over long periods of time, even with recurrent movement in adjacent tissues. On the other hand, the device should be able to penetrate the target organ without being intrusive so that tissue damage to the target organ is minimal. The device should have a small volume and a robust construction for practical medical applications.